Pulse stretcher using secondary emission tube and output amplitude regulating feedback



June 23, 1964 c. M. CLARK 3,138,758

PULSE STRETCHER USING SECONDARY EMISSION TUBE AND TUDE REGULATINGFEEDBACK OUTPUT AMPLI Original Filed June 28, 1957 3 Sheets-Sheet 1 h n4 4 K R 2 R C R H O K R W RPm A F E T f S A E U .AE B F LRIFC EER U uEWR mwE DP P N D m L m E M E G F A l G 3 E E 1 R TR 9 R R t w W E U E 4 Es T 0 n T 5 T R I /$N LP V PA DL w U U M M m w w A W P O P E E E c s mRR RA INVENTOR CALVIN M. CLARK BY M 41 MM A ORNEYS June 23, 1964 c. M.CLARK 3,138,758

PULSE STRETCHER USING SECONDARY EMISSION TUBE AND OUTPUT AMPLITUDEREGULATING FEEDBACK Original Filed June 28, 1957 3 Sheets-Sheet 2 5 N 8?K km I R I I i i Z a 9:350 50 0 655 25050 6528 35 $5050 Biz. c 513mm..WM R Emma K 525 3 5". w E 0 05580: A v m m: m2; 20E m A L 1 o2+ A E o@A M 4 c N3 m M o: .v 523 v 2w m m 518mm. 20E h \fi A m m9 w l w: DD) Q1111 k W FL kK M" I k: W! m an r WP lL R W w W Z A A 2 H zuu wm 5m v? AA3 amt-(43002 u Q w m2; oh I: muQOEk 3 #8 m9 NS 92 i K wm .3 oom+ m3 5 jJune 23, 1964 c. M. CLARK 3,138,753

PULSE STRETCHER USING SECONDARY EMISSION TUBE AND OUTPUT AMPLITUDEREGULATING FEEDBA Original Filed June 28, 1957 3 Sheets-Sheet 3 L TOTIME MODULATED PULSE GENERATOR FROM PULSE STRETCHER FROM SWEEP GENER.

ASS-

FIG. 3

INVENTOR CALVIN M. CLARK BY 4% MJUM A ORNEYS United States Patent PULSESTRETCHER USING SECONDARY EMI SEON TUBE AND OUTPUT AMPLITUDE REGU-LATING FEEDBACK Calvin M. Clark, Fullerton, Calif assignor to CaliforniaResearch Corporation, San Francisco, Calif., a corporation of DelawareOriginal application June 28, 1957, Ser. No. 668,781, now Patent No.2,968,724, dated Jan. 17, 1961. Divided and this application Sept. 28,1959, Ser. No. 842,766

1 Claim. (Cl. 328-68) This application is a division of my copendingapplication Serial No. 668,781, filed June 28, 1957, for Pulse HeightAnalyzer, now Patent No. 2,968,724, issued Ian uary 17, 1961. Thisinvention relates to a method and apparatus for converting an electricalpulse of short duration into an electrical pulse of predetermined longerduration while retaining the amplitude of the longer pulse pro portionalto the original amplitude of the shorter pulse. This invention isparticularly useful in a Well logging apparatus where pulses ofextremely short duration are detected Within a well bore many feet belowthe surface of the earth and Where the information contained in thesepulses must be transmitted to the earths surface along a well loggingcable of limited frequency and power characteristics.

In logging of earth formations traversed by a well bore it is frequentlydesirable to be able to transmit high-fre quency forms of intelligence,such as electrical pulses representing gamma ray energies which are ofvery short duration and very rapidly succeeding each other, over a cableof relatively limited frequency and power transmission characteristics.In particular, it is highly desirable to be able to transmit theelectrical pulses corresponding to the individual 'y rays generatedinstantaneously upon interaction of neutrons with nuclei of constituentelements of an earth formation lying several thousand feet below theearths surface. However, this desire has been found so difficult thatthe solutions suggested for field practice heretofore have requiredeither that the recording be performed blind, by means such as cameraand film within the well bore adjacent the logging tool, or that thesignal be transmitted over commercially unavailable coaxial cables. Asparticularly explained in the copending application of Delmar O.Seevers, Serial No. 433,244, filed May 28, 1954, now Patent No.2,802,951, which is assigned to the assignee of the present application,these previously known methods of transmitting high-frequency forms ofintelligence have been considered so unattractive that they haveretarded or prevented the field use of spectral analysis of earthformations by means of 'y ray energies. As further explained in the saidSeevers application, the intelligence originating in high-frequency formmay be transmitted over a standard well logging cable of limitedfrequency and power transmission characteristics by conversion of theelectrical pulses, corresponding to the individual energy of each 'y raydetected by a scintillation crystal and photomultiplier tubecombination, to an electrostatic charge in a cathode-ray tube having anelectrostatic charge modifiable storage surface, then assigning aparticular location or position on the storage surface for storage ofsaid charges or pulses in accordance with the energy of each 'y ray andsubsequently reading out the stored information for transmission inlow-frequency form.

In my copending application, Serial No. 451,525, for Circular ScanningSystem for Recording Nuclear Energy Spectrum, filed August 23, 1954, nowPatent No. 2,856,- 537, and assigned to the same assignee as the presentapplication, the advantages provided by polar coordinate storage of the'y ray spectrum have been described. The

ice

system of my copending application employs an electrostatic deflectiontype cathode-ray storage tube. The polar coordinate storage system ofthat application requires the conversion of the intelligence into itssinecosine coordinates to produce the polar coordinate deflectionpattern on the storage surface of the storage tube.

Present commercially available electromagnetic deflection typecathode-ray storage tubes provide substantially improved resolution overthe best electrostatic tubes. The improved resolution increases thestorage capacity of the tube by the square of the resolution improvementdue to the polar type distribution of the information on the storagesurface, thus providing a much more desirable and efficient storagedevice.

The objects and features of the invention will be readily apparent tothose skilled in the art from the specification and appended drawingsillustrating certain preferred embodiments in which:

FIG. 1 is a schematic representation of the logging tool as Well as thetransmitting and recording equipment incorporating the method of thepresent invention as applies to the logging of 7 ray energy spectra.

FIG. 2 is a schematic circuit diagram of the pulse stretcher and triggergenerator of the present invention.

FIG. 3 is a schematic circuit diagram of the amplitude comparator of thepresent invention.

Referring now to the drawings, and in particular to FIG. 1, there isillustrated a preferred form of apparatus adapted to utilize the methodof transmitting intelligence originating in high-frequency form over awell logging cable 11 of limited frequency and power transmissioncharacteristics. As shown, a logging sonde 12, wherein the earthformation analysis and signal generating equipment are located, isadapted to traverse a well bore 13 while supported on the lower end ofthe logging cable 11. The Well logging cable, it should be understood,must have considerable structural strength in order to support therelatively heavy logging sonde 12, as well as several thousand feet ofits own length.

In order for the cable to be sufiiciently small in diameter to allow forease in handling in field operations as well as the strengthrequirements, the power and frequency characteristics must be sacrificedfor the strength and handling requirements. In practice the electricalpower is restricted to the order of about watts at not over about 200volts. Accordingly, when it is desired to be able to handle and transmitinformation representing individual 'y ray quanta, it has been founddifiicult to transmit a sufficient amount and quantity of datacorresponding to a large number of channels of pulses of widely varyingmagnitudes to the earths surface so that a useful record may be madesimultaneous with the running of the logging sonde 12 in the well bore13. This is due primarily to the fact that these pulses are each of suchshort duration and follow in such rapid succession that a high-frequencytransmission system would be required even when the total amount ofinformation is limited.

In the embodiment of the invention illustrated in FIG. 1, the 'y rayquanta to be stored on the storage surface 14 of the cathode-ray tube 15arise from neutron bombardment of the earths formation, such as 21, byneutrons from source 22, which are captured by nuclei of constituentelements within the formation 21.

The neutron source 22, which may be a polonium beryllium source to givehigh neutron and low 'y ray production, is embedded within a shield 23,such as bismuth to reduce the number of 'y rays entering the formation,and further to reduce the number of 7 rays of low energy returning fromthe formation. The individual neutron capture 'y ray quanta,characteristic of the individual nuclei in the formation are thendetected by a scintillation counter including a crystal 24, such assodium iodide activated by thallium and a directly coupledphoto-multiplier tube 25. The scintillation counter further includes apreamplifier 26 wherein all electrical pulses from the photo-multipliertube, as actuated by the crystal 24, are made substantially equal inlength and proportional in magnitude to each of the 'y ray quantadetected by the counter. As shown, the scintillation counter combinationof crystal 24, photo-multiplier tube 25 and preamplifier 26, ispreferably enclosed within a further shield 27 constructed of boron inorder to prevent thermalized neutrons from entering the detector andfurther enclosed within a Dewar flask 28 to prevent the extremes intemperatures encountered within the well bore from causing thegeneration of random pulses in the photomultiplier tube. The electricalpulses from the scintillation counter are of exceptionally shortduration (about .25 microsecond in duration) and, when developed from ahigh intensity neutron source as herein disclosed, result in as many asone million counts per second being generated in the electrical circuit.Accordingly, for a direct transmission system of these pulses to thesurface, a band width of at least one megacycle, and preferably more,would be required.

The foregoing frequency requirements in view of the aforementionedlimited power and frequency characteristics of the cable 11 require thatthe electrical pulses from the scintillation counter be converted to amore usable form while still retaining informational characteristics ofeach of the 'y ray quanta detected by the counter. To accomplish thispurpose the logging sonde 12 includes further circuitry for theselection and conversion of the short duration electrical pulses intomore easily handled waveforms for transmission to the earths surfacewith a minimum amount of modification before application to the storagesurface 14 of the cathode-ray storage tube 15. This circuitry includes adiscriminator 29, a gate generator 30 and a pulse stretcher 31 fed fromthe preamplifier 26 wherein the electrical pulse from the scintillationcounter is converted from a sharp pulse to a stretched rectangular pulseproportional in amplitude to the amplitude of the input pulses. Theoutput of the pulse stretcher 31 is fed to an amplitude comparator 32where the rectangular waveform is compared to a linear sawtooth voltagedeveloped in linear sweep generator 33 to produce a pulse signal havinga time of initiation determined by the amplitude of the electricalsignal and, in that manner, the energy of the 'y ray detected. Theoutput of the amplitude comparator is fed to a rectangular gategenerator 34 of conventional bi-stable multivibrator design which isthereby returned to its normally OFF state after having been previouslyturned ON by a trigger pulse from the pulse stretcher; a pulse is thusformed having its duration or width determined by the energy level ofthe 'y rays detected by the scintillation counter. The particulars ofconstruction and operation of the foregoing down hole components of thelogging tool of the present invention will be more fully explainedhereinafter along with the interconnection and coordination of each withthe other.

The output of the rectangular gate generator 34 will be transmittedalong the cable 11 to the components of the logging tool located at theearths surface. The output of the rectangular gate generator will be arectangular waveform having a time duration proportional to the energylevel of the 'y ray retected, and, as such, may be used to developenergizations for the storage tube 15. The storage tube 15 is providedwith a rotating magnetic deflection coil 41 and a motor 42 is providedfor rotating the deflection coil around the electron beam of the storagetube at a constant speed. The individual bits of information derivedfrom the energy of the 'y rays detected by the scintillation counterwill be stored on the surface 14 of the storage tube 15 as discretetraces as shown in FIG. 2 with the radial distance of these traces fromthe center of the storage surface proportional to the energy of the 'yrays detected. To accomplish this proportionality the rectangularwaveform output of the rectangular gate generator 34 transmitted alongcable 11 to the components of the logging tool at the earths surface isfed to a pulse shaper 43 such as the well-known Schmitt trigger circuitwhere it is reformed for energization of the actuating circuits for thestorage tube 15. The pulse shaper 43 provides a pair of outputsconstituting a rectangular waveform being fed to a linear sweepgenerator 44 for initiation of the radial sweep to be applied to therotating deflection coil and the sharp pulse output of the triggercircuit 43 being fed to a pulse generator 45 to develop a gating pulsefor controlling the electron beam of the storage tube 15. Through theapplication of a linear sweep current derived from the sweep voltage tothe deflection coils 41 of the storage tube and the sharp pulse gatefrom the pulse generator 45 to the electron control elements of thestorage tube the information related to the detected 'y ray will bepositioned on the storage surface 14 of the storage tube 15 in the formof discrete traces 46 radially disposed from the center of the storagesurface a distance proportional to the energy of the 'y ray detected. Asa preferred manner of operation, the radial deflection of the electronbeam of tube 15 is such that the low energy 'y rays will be representedby traces near the outer periphery of the storage surface 14.correspondingly, the higher energy traces will be positioned nearer thecenter of the target surface.

In the form herein specifically shown, this is accomplished in the downhole circuitry wherein the minimum original pulse amplitude results in amaximum pulse width for transmission through the logging cable. Thistime modulation is stated by the following expression:

At: T-ke where At=time duration of the time modulated pulse.

T=predetermined maximum time for analysis of pulses of the lowest signallevel.

k=a proportionality factor.

e=the energy of the 'y ray.

In an alternative form, not herein specifically shown, the timemodulation of the transmitted signal may take the form expressed as:

At=ke with the symbols having the same meanings as above. Theinformation transmitted in the from of Equation 2 is ultimatelyconverted at the earths surface to the form of Equation 1 forapplication to the storage tube.

A pulse counter 49 is provided to be actuated by the pulse generator 45and, as has been hereinbefore indicated, the read and write controlswitch 47 may be either automatically operated by the pulse counter toprovide a readout arrangement after a predetermined number of pulseshave been stored on the storage tube or the read and write controlswitch may be manually operated to perform the same function. Thereadout amplifier 48 drives a readout recorder 51 having a stylus 52 forindicating on a record paper 53 the information stored on the storagesurface 14. The record paper 53 is preferably driven by a constant speedmotor 54.

A depth indicator 55 mechanically driven by the cable reeling mechanismsprovides an output to the readout recorder 51 for indication on therecord 53 of the location within the well bore of the 'y ray spectrabeing recorded.

The circuits for converting the output of the scintillation counter frompulses of high frequency and short duration will now be described. Ashas been previously explained the cable 11 is incapable of handling thepulses in the form derived from the scintillation counter necessitatingthe conversion of these pulses to a more usable form. The circuitdiagrams of FIGS. 2 and 3 illustrate the pulse stretcher and amplitudecomparator circuits for converting the scintillation counter output froma pulse having an amplitude proportional to the energy of the 'y ray itrepresents to a pulse having a time duration proportional to theamplitude of the detected 'y ray.

The output pulse of the preamplifier 26 passing through thediscriminator 29 is fed to the pulse stretcher circuit including adifference amplifier circuit '71 comprising a pair of vacuum tubes 72and 73. The pulse, with its amplitude proportional to the energy of the'y ray detected, is applied to the control grid 74 of tube 72 and afeedback signal is applied to the control grid 75 of tube 73 derivedfrom a source to be explained hereinafter. The anode 76 of tube 72 isconnected by conductor 77 to the control grid 78 of a vacuum tube 79 ofa two-tube series amplifier 81 having as the second stage a vacuum tube82, the anode 83 of tube 73 being connected by a conductor 84 to thecontrol grid 85 of the tube 82. Conduction through tubes 72 and '73 iscontrolled by constant current device tube 80 such that the totalcurrent through the two tubes remains constant to be proportionedbetween the two tubes. The cathode 86 of tube 82 is connected to anegative potential source at conductor 87 and the anode 88 is connectedto the cathode 89 of tube 79. Anode 91 of tube 79 is connected through asuitable voltage dropping resistor 92 to a source of positive potentialat conductor 93. The junction of anode 88 of tube 82 and cathode 89 oftube 79 is connected by conductor 94 to the cathode 95 of a vacuum tube96 of the secondary emission type. Tube 96 has an anode 97, a dynode 98,a control grid 99 and other conventional elements usual with a tube ofthe type shown. The cathode 95 of tube 96 is connected through a loadresistor 100 to the anode 101 of a tube 102 having its cathode 103connected through circuit means to the negative potential at conductor87. Control grid 104, of tube 102 has applied thereto, through acoupling capacitor 105, a gating pulse from a source to be describedhereinafter, and an adjustable DC. bias from potentiometer 112 seriallyconnected with resistor 111 to provide a voltage divider networkconnected between ground and the conductor 87.

The dynode 98 of tube 96 is connected to one side of a capacitor 106 andto the anode 107 of a tube 108 the cathode 109 of which is connected tothe other side of the capacitor 106 and through circuit means to ground.The control grid 113 of tube 108 has a gating pulse applied thereto froma source, to be later described, through a coupling capacitor 114 and isfurther connected by circuit means to a source of potential positivewith respect to the cathode 109. The anode 107 is directly connected tothe control grid 115 of a vacuum tube 116 operating in acathode-follower circuit with its anode 117 connected to the positivesource at conductor 93 and its cathode 118 connected through loadresistors 119 and 121 to ground. The junction of resistors 119 and 121is connected by conductor 122 to the control grid 75 of the tube '73applying the previously defined feed back to that tube.

Referring again to tube 96, its anode 97 is connected through theprimary 123 of a transformer 124 to the positive potential at conductor93. The secondary 125 of the transformer is connected through circuitmeans to the cathode 126 and control grid 127 of a vacuum tube 128operating as a so-called bootstrap cathode follower amplifier with thecathode 126 connected through load elements 129 to ground and the anode1131 connected to the positive potential at conductor 93. Cathode 126 isalso directly coupled to the control grid 132 of a vacuum tube 133operating as a cathode follower and having its cathode 134 connectedthrough load means to ground and its anode 135 connected to the positivepotential at conductor 93. A load resistor 136 is pro vided in thecathode circuit and a coupling capacitor 137 is connected to pick offthis pulse for triggering the time modulated pulse generator 34 as willbe more fully described hereinafter.

The circuit elements just described constitute a pulse stretcher andtrigger generator for the pulse height analyzer and operate in thefollowing manner:

Pulse stretching is accomplished by charging capacitor 106 to a positivepotential with current flowing through the dynode 98 of tube 96; theeventual charge is made proportional to the peak value of the inputpulse to tube 72 by means of the feedback circuit through conductor 122.

With no input signal, the capacitor 106 is kept discharged by thenormally conducting tube 108. The tube 108 is driven into cut-off beforean input pulse to the capacitor 106 reaches its maximum height by anegative gating pulse through coupling capacitor 114 derived from theinput discriminator circuit 29 and gate generator 30 fed through thepreamplifier 26. In this manner the pulse stretcher circuit provides anoutput only with input signals of a predetermined voltage level.

Tube 96 is biased at, or close to, cut-oil by the voltage drop acrossthe load resistor of tube 102 in its cathode circuit with the controlgrid 99 connected to be negative with respect to the cathode 95. Thecathode potential of tube 96 is adjusted by variable resistor 110, inthe control grid circuit of tube 79, such that the steady statecathode-dynode potential results in an electron secondary emission ratioof the dynode in tube 96 near unity. This control of the cathodepotential is accomplished by the DC. ocupling of the grid 78 of tube 79to the anode of tube 72 through a voltage divider network includingvariable resistor 110. Adjustment of resistor will change the DC.voltage on grid 78 and, because tube 79 is essentially a cathodefollower, the cathode 89 will move with the grid 78. The cathode 95 oftube 96, being directly connected to cathode 89 of tube 79 will be movedto the same adjusted potential. In the foregoing manner the potentialdifference between the dynode 98 and the cathode 95 is adjusted tomaintain the electron secondary emission ratio of the dynode near unityor a net dynode current of zero. The current through tubes 79 and 82controls the potential on the cathode 103 of tube 104 and this, alongwith the gridbias potentiometer 112, controls the conduction throughthat tube and therefore the grid-cathode voltage of tube 96.

When an input pulse appears at control grid 74 of tube 72, the amplifiedsignal is applied push-pull to the two tube series amplifier tubes 79and 82. The amplified signal through tube 72 is fed to the control grid78 of tube 79 and the amplified signal through tube 73 is fed to thecontrol grid 85 of tube 82. The initial amplified signal from tube 72drives the grid 78 of tube 79 negative toward cut-oil thus reducingcurrent flow through tube 79 and allowing cathode 89 to move nega'tively. At this time tube 82 is caused. to conduct more heavily by thepositive signal on grid 85. Current flow through tube 82 then passesthrough tube 96 because of the conduction condition of tube 79. Theheavier current through tube 82 increases the voltage drop across theresistor in the common cathode circuit of tube 82 and tube 102; thuselfectively cutting off tube 102 and further decreasing the currentthrough tube 79 thus reducing the negative bias between grid 99 andcathode 95 of tube 96. The cathode 95 of tube 96 is now sulficientlynegative with respect to the dynode 98 that the flow of electronsthrough the tube causes a secondary emission ratio greater than unityand a net electron current will leave the dynode and be acceleratedthrough tube 96 tothe anode 97. These electrons are available to thedynode 98 from the capacitor 106 and thus a rapid charging of capacitor106 is effected. The charge on capacitor 106 and the potential of thedynode 98 permits increased current flow through tube 116 causing apositively increasing voltage to appear across the load resistors 119and 121 in the cathode circuit thereof. This positively increasingvoltage is fed back by conductor 122 to the control grid 75 of tube 73to increase the current flow through this tube and, because of theconstant current control of tube 80, decrease the current through tube72. The increase in current through the load resistor in the anodecircuit of tube 73 decreases the voltage at grid of tube 32 causing adecrease of current through tube 82 and resistor 120, allowing tube 102to begin to conduct again. At the same time, the decrease in currentthrough tube 72 allows the grid 78 of tube 79 to become more positive topermit increased current fiow through the tube 79 and cause cathode 89to return to a more positive condition; this increased current in tube79 and the increased current through tube 102 changes the bias betweencathode and grid 99 of tube 96 to return the grid 99 to a controllingpotential.

The above voltage movements of the various elements of tubes 72, 73, 79,82, 102 and 116 are effected extremely rapidly thus giving capacitor 106a very rapid charge and a sharp rise to the waveform as shown adjacentto tube 116. Capacitor 106 continues to charge until the voltage fedback by conductor 122 to grid 75 of tube 73 equals the input signalvoltage to grid 74 of tube 72 at which time there will no longer be anerror signal between tubes 72 and 73 and therefore no error signal tothe tube 79 and 82 to keep tube 96 conducting as above. Capacitor 106will remain charged until the gating pulse from gate generator 30through capacitor 114 is removed from the grid 113 and during thisperiod tube 116 will continue to conduct to produce a stretched pulsehaving an amplitude proportional to the amplitude of the input signalfrom the preamplifier 26. When tube 108 is gated into conduction,capacitor 106 will be rapidly discharged and tube 116 will terminate itsconduction.

The termination of current flow through tube 96 when there is no errorto tubes 72 and 73 initiates a pulse in the secondary of transformer 124for application between the grid 127 and the cathode 126 of tube 128.The output of tube 128 is fed to cathode follower tube 133 to provide alow impedance pulse for triggering ON the time modulated pulse generatorthrough capacitor 137. The positive rectangular waveform output of thetime modulated pulse generator, as hereinafter defined, is fed to thecontrol grid 104 of tube 102 through coupling capacitor 105 causing asurge of current through tube 102 to prevent the pulse stretcher frombecoming responsive to a second input signal during the period it isdeveloping a pulse in response to an input signal from the preamplifier26.

The output stretched pulse from the signal indications derived throughthe circuits of FIG. 2 is fed to the amplitude comparator circuit 32 forcomparison to a linear sweep signal from sweep generator 33. In thecomparator circuit a signal is produced having a time of initiation,relative to the sweep start, determined by the energy of the 'y ray ofthe original signal indication from which the stretched pulse wasproduced. The amplitude comparator circuit 32 constitutes adiscriminator 141 including vacuum tubes 142, 143 and 144 with tube 144operating as a constant current device for tubes 142 and 143. Thestretched pulse output from tube 116 of FIG. 2 is fed to the controlgrid 145 of tube 143 and a linearly decreasing sawtooth type sweep isfed to grid 146 of tube 142 from sweep generator 33. The anodes 147 and148 of tubes 142 and 143, respectively, are connected through equal loadresistors to the source of positive potential at conductor 93 and a pairof equal series resistors 149 and 151 interconnect the two anodes. Atthe junction of the two resistors a feedback signal is picked offthrough capacitor 152 and fed to the control grid 153 of tube 144 toprovide a negative feedback helping to keep the reference value of theoutput signal constant, as will be seen hereinafter. The control grid153 is biased to a desired constant current operating condition by theunbypassed resistor in the cathode circuit and the voltage dividernetwork of resistor 154 and 155.

Further stabilization of the reference value of the output signal isprovided by the capacitor coupling the screen grids and cathodes of bothtubes 142 and 143.

A pair of diodes 156 and 157 are connected in series with their anodesback-to-back, between the junction of the resistors 149 and 151 and theanode 148 of tube 143 so that the signal output of the discriminatorcircuit may be picked off at the junction of the two diodes by conductor158.

The output of the discriminator circuit is applied through conductor 158to a differentiating circuit comprising capacitor 159 and resistor 161shunted by diode 162. The differentiated signal is applied to thecontrol grid 163 of vacuum tube 164 operating as a conventionalamplifier stage having its amplified output fed to additional amplifyingstages in tubes 165 and 166 operating as cathodecoupled amplifiers. Theoutput of tube 166 is fed through a differentiating circuit of couplingcapacitor 167 and resistor 168 for triggering OFF the time modulatedpulse generator 34 as will be more fully described hereinafter.

The operation of the foregoing circuits is as follows: The control grids145 and 146 of tubes 143 and 144, respectively, are biased such that ina quiescent condition tube 142 is conducting heavily causing tube 143 tobe cut off, this because of the lower positive potential on grid 145 andthe constant current control of tube 144. The quiescent current throughtube 144 is determined by resistor 150 and the biasing voltage dividernetwork of resistors 154 and 155. The baseline of the negative goingsawtoothed sweep input to tube 142, as shown adjacent to the grid 146from the sweep generator 33 of conventional design, is more positivethan the positive peak of a stretched pulse input to tube 143, as shownadjacent to grid 145, from the previously described pulse stretcher 31,so that tube 143 cannot conduct until the sweep on grid 146 of tube 142has reduced this grid voltage to within about 5 volts of the voltage ongrid 145 of tube 143. When the voltage on the two grids becomesubstantially equal both tubes Will conduct and there will besubstantially no difference voltage between the anodes 148 and 147 ofthe two tubes and therefore no signal output. Before this condition ofsubstantial equality between grid voltage and when tube 143 is cut off,the cathodes of tubes 142 and 143 are following the negatively goingsweep on the grid 146 and the voltage at the junction of the tworesistors 149 and 151 remain substantially constant as do the voltagesat the anodes of tubes 142 and 143; this is the result of the action ofthe constant current tube 144 and the maintenance of a constantscreen-cathode potential for tubes 142 and 143 by the screen-cathodecoupling capacitor 160.

Any variation in the reference voltage at the junction of the tworesistors 149 and 151 is applied to the grid of the constant currenttube 144 in a manner such that as the sweep signal on grid 146 movesnegatively the bias on tube 144 is changed so as to maintainsubstantially constant the current therethrough, thus maintaining thereference point voltage substantially constant. The potential at thejunction of the two diodes 156 and 157 will be held effectively at thereference junction voltage until the signals on grids 145 and 146 causethe voltages at anodes 148 and 147, respectively, to become equal. Withthe anode 148 potential positive relative to the reference junctionpoint, the forward resistance of diode 156 is very small relative to theback resistance of diode 157 and the voltage drop across diode 156 isvery low compared to that across diode 157. When tube 143 begins toconduct, as the signals on grids 145 and 146 become nearly equal, thecurrent through its load resistor will result in the anode voltagemoving in a less positive direction while the simultaneous decrease ofcurrent through tube 142 and its load resistor will result in the anodevoltage of tube 142 moving in a more positive direction; the centralreference voltage at the junction of resistors 149 and 151 is thereforeheld constant. At the instant the anode voltage of tube 143 swingsnegative with respect to the reference junction voltage, when thepotentials on grids 145 and 146 are substantially equal, the relativeresistances of diodes 156 and 157 will reverse, and the potential attheir junction will move with that of the anode 148 of tube 143. Thevoltage at anode 148, and at the diode junction, continues to swingrapidly negative with the sawtooth signal until tube 142 is cut ott andtube 143 conducts fully. The anode potentials remain at these new levelsuntil the signals are removed from the two grids 145 and 146. Theresulting negative step signal from the diode junction is ditferentiatedby capacitor 159 and resistor 161 and applied to control grid 163 oftube 164.

The signal to the grid 163 will be a single ended negative pulse throughthe provision of the bypass diode 162 and the diodes 156 and 157 whichallow for the rapid removal of the charge on capacitor 159 and for a lowresistance path around the resistor 161 across which the signal for grid163 is developed.

The output of tube 164 is applied to the grid of tube 165 in theconventional manner. Tubes 165 and 166 constitute a cathode-coupledclipper-limiter amplifier with the clipped signal taken from the cathodecircuit of the tube 165 being applied to the cathode of the groundedgrid amplifier tube 166. The amplified and limited output of tube 166 istaken from the anode circuit and differentiated through couplingcapacitor 167 and resistor 168 to produce a sharp positive pulse for thetriggering OFF function in the time modulated pulse generator.

The time modulated pulse generator 34 is a conventional bi-stablemultivibrator circuit for generating a rectangular pulse output having atime duration determined by the triggers from the pulse stretcher andthe amplitude comparator. The discriminator circuit 29 selects theelectrical pulses produced by the photo-multiplier tube 25 fallingwithin a predetermined energy range thus providing an initial screeningagainst undesired low energy information. A trigger output of thediscriminator 29 initiates the operation of a bi-stable multivibratorcircuit of the gate generator 30 to activate the pulse stretcher 31. Theselected pulses from the discriminator 29 are fed to the pulse stretcher31 for widening while retaining their relative amplitudes.

The widened pulses from the pulse stretcher 31 are then fed to theamplitude comparator 32. Another output of the pulse stretcher 31, aspreviously described, is used for triggering ON the time modulated pulsegenerator. The output of time modulated pulse generator, in addition tobeing fed up the logging cable 11, actuates the sweep generator 33 andenergizes a blocking circuit in the pulse stretcher 31 to eliminateinterference from other signals during a period [that a selected pulseis being stretched. The linear sweep generator generates anegative-going linear sawtooth waveform which is the other input to theamplitude comparator 32.

The output of the amplitude comparator 32 is a pulse signal indicatingthe instant of equality of the stretched pulse 31 and the sawtoothwaveform. This output is fed to the time modulated pulse generator 34 toterminate or trigger OFF the aforementioned rectangular pulse.Conneotions are provided, as shown in FIG. 1, between the rectangulargate generator 34, the linear sweep generator 33 and the pulse-stretcher31 so that, upon termination of the rectangular pulse from the gategenerator 34, the pulsestretcher 31 will be prepared to handle asubsequent signal from the preamplifier 26 and the linear sweepgenerator 33 will be prepared to initiate a new sawtooth waveform.

While a certain preferred embodiment of the present invention has beenspecifically disclosed, it is understood that the invention is notlimited thereto, as many variations will be readily apparent to thoseskilled in the ant and the invention is to be given its broadestpossible interpretation within the terms of the following claim.

I claim:

A pulse stretcher circuit for converting an input electrical pulse ofshort duration into an electrical pulse of predetermined longer timeduration having its amplitude proportional to the amplitude of therespective shorter pulse comprising in combination an electronic tubehaving an anode, a cathode, a control electrode and a secondary emissiondynode, first circuit means for initiating electron flow through saidelectronic tube with initiation of said short pulse, said circuit meansincluding means for reducing the voltage difference between said cathodeand control grid to reduce the control exerted by said control grid onsaid electron flow at said time of initiation and for increasing thevoltage difference between said cathode and dynode to increase electronflow through said tube, said electron flow being directed toward saiddynode to cause release of electrons from said dynode by said secondaryemission, means for charging a capacitor by said secondary emissionelectrons, second circuit means including a feedback circuit responsiveto the charge on said capacitor and operative on said first circuitmeans for increasing the bias betwen said cathode and control grid toincrease the control exerted by said control grid on said electron flowand for reducing said voltage dilterence between said cathode and dynodewhen said capacitor has attained a charge proportional to the amplitudeof said short pulse, timing circuit means for discharging said capacitorafter said predetermined longer time duration, and means for sensing thecharge on said capacitor to provide an output signal having theamplitude of said input pulse and a predetermined time duration.

References Cited in the file of this patent UNITED STATES PATENTS2,767,311 Meyer Oct. 16, 1956 2,803,748 Kalibjian et a1 Aug. 20, 19572,922,879 Vogt Jan. 26, 1960 2,954,466 Campbell Sept. 27, 1960

